There has recently been an idea that a virtualization technique and a thin provisioning technique are combined with each other to allow a storage region to be managed in smaller units than logical volume (storage tier management technique).
In the storage tier management technique, a storage region in a virtual logical volume (hereinafter referred to as a virtual volume) based on thin provisioning is divided into a plurality of partial regions (hereinafter referred to as virtual regions). Then, to which of the tiers a storage device including a substantive storage region (hereinafter referred to as an actual region) to be assigned belongs is determined for each virtual region. For example, according to Patent Literature 1, the storage region in a virtual volume and the storage region in a physical storage device are divided into at least two virtual regions and at least two actual regions, respectively. One actual region is assigned to each virtual region. Furthermore, for each actual region, the number of accesses from a host computer and the allowable range of the access count are recorded. Data in an actual region with the access count exceeding the allowable range is migrated to another actual region in the storage device in accordance with the access status.